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Reductive sorption of Cr(VI) from aqueous solution on dehydrated date palm leaflets.

Introduction

Industrial processes including mining operations, power generation, leather tanning, paint and pigment manufacturing, electroplating, electronic device manufacturing, canning, metal finishing, and chromate preparation are considered as potential sources for chromium pollution in wastewater. Chromium occurs in aqueous systems as both Cr(III) and Cr(VI). Cr(III) apparently plays an essential role in plant and animal metabolism when emitted in low levels in the environment. However, Cr(VI) is toxic to bacteria, plants and animals [1]. The International Agency for Research on Cancer has determined that Cr(VI) is carcinogenic to humans [2]. The maximum concentration limit for Cr(VI) for discharge into inland surface waters is 0.1 mg/l and in potable water is 0.05 mg/l [3].

Cr(VI) is typically present as anion and its precipitation is not usually practical. Instead, the anionic species of Cr(VI) are typically reduced to Cr(III) in acid medium and then precipitated as chromic hydroxide by using NaOH [4]. However, it is a process which can give an incomplete removal, has a high chemical requirement and produces voluminous toxic sludge which may pose disposal problems [5]. Other methods used for the removal of chromium ions include reverse osmosis, evaporation and ion exchange [6]. These methods have been found to be limited, since they often involve high capital and operational costs and may also be associated with the generation of secondary wastes which present treatment problems. Moreover, these methods are not effective at metal concentrations ranging from 1 to 100 mg/l [7].

Sorption of Cr(VI) from wastewaters has been recently emphasized by the use of various sorbent materials such as activated carbon [5,8-10], biogas residual slurry [11], boehmite [12], used tyres and sawdust [13], chitosan [2], sphagnum moss peat [14], Mg-Al-C[O.sub.3] hydrotalcite [15], bauxite [16], polymer material [17], leaf mould [18], grape stalks and yohimbe bark [19] and modified flax shive [20]. Reduction of Cr(VI) to Cr(III) was observed with different sorbents [9,14, 18-23] at low pH values. Using activated carbon, Perez-candela et al. [8] found that Cr(VI) is reduced at pH < 3 while Huang and Wu [22] found that the reduction occurs at pH < 6. In another study, using modified flax shive, reduction of Cr(VI) took place at pH [less than or equal to] 7 [20].

Date palm (Phoenix dactylifera L.) is the primary crop in Oman occupying 49 % of cultivated area and 82 % of all fruit crops grown in the country [24]. Date palm has maintained its domination of Oman fruit trees with an estimated 7.8 million trees grown in 2005 [24] and about 8.8 million trees by 2015. Palm leaves and leaflets are produced in huge quantities in Oman every year with little or no use. In this paper, palm leaflets were treated with sulfuric acid to produce a carbonaceous material that is capable of reducing Cr(VI) to Cr(III). The capability of the produced sorbent to remove Cr(VI) from aqueous solution in terms of kinetics and equilibrium was investigated and reduction sorption processes are discussed.

Experimental

All the chemicals used were of analytical grade. Dry date palm leaflets were collected from a local farm and washed with distilled water to remove dirt, dust and any superficial impurities. The leaflets were put into trays and left to dry in open air at room temperature to constant weight. The leaflets were then cut to pieces (1-2 cm) using a scissor.

Sorbent Preparation

Clean air-dried palm leaflets (50 g) were weighed in a 500 ml clean dry beaker. 200 ml of 13 M sulfuric acid were added to the palm leaflets and the mixture was heated to 170-175 [degrees]C in 25 minutes with occasional stirring. The temperature was kept in this range (170-175 [degrees]C) for 20 minutes. The resulting black mixture was allowed to cool, and then filtered using a Buchner funnel under vacuum. Spent sulfuric acid was filtered off and the carbonized material was washed several times with deionized water and stored under dilute acidic conditions (dilute sulfuric acid, pH 1.5-2) to avoid any bacterial effect. Before use for Cr(VI) sorption, a sample of the carbonized product was washed in Gooch crucible until the wash water did not show a change of methyl orange color. The sample was transferred to a Gooch crucible and left under suction for 30 minutes. Suitable samples of the carbonaceous wet sorbent were then used in sorption experiments and a sample of ~1 g was separated to measure the moisture content using oven drying at 105 [degrees]C to constant weight. Sorbent yield, moisture and ash contents were found to be 95, 80 and 11.6 %, respectively.

Preliminary experiments showed that if the carbonaceous sorbent, produced via sulfuric acid treatment, was dried, its efficiency for Cr(VI) removal and total chromium sorption decreases with a slower kinetics. This is probably related to shrinkage and compaction of the sorbent on drying giving narrower pores for the diffusion of the Cr(VI) ions [20]. Accordingly, the wet sorbent was selected, in the present study, for testing the reduction and sorption processes of Cr(VI).

Sorption Experiments

A stock Cr(VI) solution (1000 mg/l) was prepared in deionized water using potassium dichromate ([K.sub.2][Cr.sub.2][O.sub.7]). All the working solutions were prepared by diluting the stock solution using deionized water. Batch experiments were carried out by mixing 50 ml of Cr(VI) solution (100-500 mg/l) at initial pH (1.8-7.0), at 25 [degrees]C, with 0.50 g of the wet sorbent (equivalent to ~0.1 g on dry basis) in a shaker incubator (100 rpm). The isotherm studies were carried out at different initial pH (1.8-7) for initial Cr(VI) concentration (100-500 mg/l). The pH was adjusted by adding few drops of 0.1 M sulfuric acid or 0.1 M sodium hydroxide before the addition of the pre-weighed sorbent. After the equilibrium time was reached, the final pH was recorded and aliquot of supernatant was withdrawn and analyzed for Cr(VI) and total chromium.

In the kinetic experiments, ~2 g of the wet sorbent (equivalent to ~0.4 g on dry basis) were mixed with 200 ml of Cr(VI) (100 mg/l) at initial pH 2.0, 2.8 and 4.0. At different time intervals, the pH value of the reaction solution was recorded and aliquot of supernatant was withdrawn for the analysis of Cr(VI) and total chromium. For Cr(VI) removal, that is based on the decrease in Cr(VI) concentration with time, two kinetic models were used: pseudo first order and pseudo second order models [20], (Eqs. 1&2, respectively).

log[C.sub.t] = log [C.sub.o] - [k.sub.1]t/2.303 (1)

1/[C.sub.t] = 1/[C.sub.o] + [k.sub.2]t (2)

[C.sub.t] is the Cr(VI) concentration at time t, [C.sub.o] is the initial concentration of Cr(VI) while [k.sub.1] and [k.sub.2] are the pseudo first order and pseudo second order rate constants for Cr(VI) removal, respectively.

For chromium sorption, which is based on the uptake of total chromium with time, two kinetic models were tested: pseudo first order and pseudo second order models [25, 26], (Eqs. 3&4, respectively).

log ([q.sub.e]-[q.sub.t)] = log [q.sub.e] - [k.sub.1.sup.'] t/2.303 (3)

t/[q.sub.t] = l/[k.sub.2.sup.'] [q.sub.e.sup.2] + t/[q.sub.e] (4)

Where [q.sub.e] and [q.sub.t] are the amounts of chromium sorbed at equilibrium and any time per unit weight of sorbent (mg/g) respectively. [k.sub.1.sup.'] and [k.sub.2.sup.'] are rate constants for the pseudo first order and pseudo second order models, respectively. The initial sorption rate h = [k.sub.2.sup.'] [q.sub.e.sup.2].

Metal Analysis

Cr(VI) was analyzed spectrophotometrically using 1,5 diphenylcarbazide method [27] at [lambda] max 540 nm (Varian Cary 50 Conc UV-visible spectrophotometer). Total chromium was determined by atomic absorption (Varian Spectra AA 220 FS atomic absorption spectrometer). The difference in concentration between total chromium and Cr(VI) gives Cr(III) concentrations. Experimental procedure and analysis were carried out in triplicates and maximum analytical error was found to be less than 5%.

Base neutralization

Base neutralization capacity was measured by Boehm titrations [28]. Neutralization of 0.1 M sodium bicarbonate and sodium hydroxide, and 0.05 M sodium carbonate, by the dry sorbent, was studied by mixing 0.25 g sorbent portions with 50 ml of the respective solution in 100 ml Quickfit polyethylene bottles. The suspensions were flushed with

nitrogen gas to remove oxygen present. This step was essential to minimize the possibility of base-catalyzed auto-oxidation of carbonaceous sorbents in the presence of oxygen [29]. The suspensions were shaken mechanically for 72 hours covering the equilibrium time. The fall in concentration of each solution was determined by titration of an aliquot of the supernatant liquid against 0.1 M HCl.

Results and discussion

Date palm leaflets (agricultural material) possess cellulose, hemicelluloses and lignin as main components. Concentrated sulfuric acid behaves as a strong dehydrating and oxidizing agent [30]. Under the preparation conditions, carbonization with partial oxidation took place to the cellulose and hemicelluloses with partial fragmentation to the lignin components [31] resulting in a carbonaceous material loaded with surface functional groups as -COOH and -OH. The effect of hot sulfuric acid on flax shive producing a carbonaceous sorbent was studied and published earlier [31].

Effect of pH

Chromium (VI) is readily hydrolyzed in water [21]. The dominant Cr(VI) species at total chromium concentrations below 500 mg/l are hydrochromate, HCr[O.sub.4.sup.-] , and chromate, Cr[O.sub.4.sup.2-]. HCr[O.sub.4.sup.-] is the dominant species at low pH while Cr[O.sub.4.sup.2-] predominates at a higher pH environment such as natural waters [21]. The dichromate ion [Cr.sub.2][O.sub.7.sup.2-] predominates in acidic environments at concentrations higher than 500 mg/l.

Optimal pH for maximum Cr(VI) sorption was reported variable depending on Cr(VI) concentration and the sorbent used. Maximum chromium sorption on activated carbon (filtrasorb 400) occurred at pH 5-6 [22] while using granular activated carbon, maximum sorption occurred at pH 2.5-3 [9]. Other sorbents showed maximum sorption of Cr(VI) at different pH such as chitosan derivatives (pH 4) [4], sphagnum moss peat (pH 1.5-3) [14], saw dust (pH 2-3) [32] and modified flax shive (pH 2.2-2.6) [20].

In the present study, different initial pH values (pH 1.8-7) were applied for initial Cr(VI) concentration of 100-500 mg/l. As shown in Fig. 1, the optimal pH for Cr(VI) sorption is in the pH range 2.0-2.8 showing a shift in sorption maxima to lower initial pH values with the increase in Cr(VI) concentration. At initial pH below that of maximum sorption, Cr(VI) was barely detected in solution and Cr(III) was the main constituent of total chromium (Figs. 2A & 2B) showing low sorption with a slight rise in the final pH (Fig. 3). The equilibrium pH was low enough that protons compete effectively with evolved Cr(III) for binding sites on the sorbent surface, such as -COOH and -OH [31]. Accordingly, low sorption was obtained. At the initial pH of sorption maxima, total chromium detected in solution (at equilibrium) was minimal constituting Cr(VI) mostly with little or no Cr(III), Figs. 2A&2B. A large increase in the final pH was observed allowing most of produced Cr(III) to be sorbed via ion exchange or complexation [33].

At initial pH above that of the sorption maxima, as the initial pH increases (until initial pH 7), sorption capacity decreases, with an increase in the final pH and no Cr(III) was detected in equilibrium solution. Reduction of Cr(VI) is expected to decrease at such high pH due to protons insufficiency. Thus, little Cr(III) was produced and low sorption was obtained. In addition, an extent of physico-chemical adsorption of Cr(VI) is expected to take place at such high pH [32]. Fig. 3 represents examples of the pH change for three initial Cr(VI) concentrations (100, 300 and 500 mg/l).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

As shown also in Fig. 1, sorption maxima shift to lower initial pH values when higher Cr(VI) concentrations are treated. Cr(VI) reduction can be presented by Eq. 5.

HCr [O.sub.4.sup.-] + 7[H.sup.+] + 3[e.sup.-] = Cr (III) + 4[H.sub.2]O (+1.350 V) (5)

In the presence of the sorbent, as a reducing agent, the amount of Cr(VI) reduced increases with the increase in concentration of either protons or Cr(VI), or both (Eq. 5). Erdem et al. [34] found that the amount of Cr(VI) reduced per gram of siderite (at equilibrium) has increased with the increase in the initial Cr(VI) concentration at the same acid concentration. Thus, with the increase in initial Cr(VI) concentration, more protons (or lower pH values) are required for more Cr(VI) reduction. Sorption maxima appear when Cr(VI) reduction is accompanied by a large rise in the final pH allowing sorption of most Cr(III) produced. Accordingly, as initial Cr(VI) increases sorption maxima shift to lower initial pH values, however, the final pH stayed similar varying in a narrow range of 4.45-4.86 (Table 1). Similar shifts in the optimal pH of chromium sorption to lower initial pH values with the increase in Cr(VI) concentration were observed for Cr(VI) adsorption onto activated carbon [9], sphagnum moss peat [14], saw dust sorbent [32] and modified flax shive [20].

Kim and Zoltek [35] found that the optimal chromium sorption, using activated carbon, occurs when the ratio of initial protons and Cr(VI) concentration, [[H.sup.+]]/[Cr(VI)], is 1:1, however, in other studies, such ratio was 0.16-2.6 for activated carbon [9]. Such variation is directly related to the difference in reduction capability of the different sorbents for Cr(VI). In the present study, the ratio of [[H.sup.+]]/[Cr(VI)], at optimal chromium sorption, was in the range of 0.6-0.95, (Table 1). A relationship can be established between initial [Cr(VI)] and initial pH, instead of initial [[H.sup.+]], Fig. 4. The relationship appears linear with good correlation ([R.sup.2] 0.9996) and is presented in Eq. 6. Such a relationship can be useful to predict the optimum conditions for the removal of Cr(VI) under the experimental conditions.

[[H.sup.+]] = [10.sup.0.0094[Cr(VI)]] (6)

[FIGURE 4 OMITTED]

The sorption data were also presented in terms of chromium concentrations (100-500 mg/l) for each initial pH to produce a sorption isotherm and were tested for the Langmuir equation (Eq. 7).

[C.sub.e]/[q.sub.e] = 1/(b.q) + [C.sub.e]/q (7)

[C.sub.e], the equilibrium chromium concentration; q, the amount of chromium sorbed per unit weight of the sorbent required for monolayer coverage; b, the Langmuir constant.

Sorption of total chromium, follows, to some extent, the "L-type" isotherm mostly at initial pH values [greater than or equal to] 2.6. For pH values <2.6, sorption data neither show the L-type isotherm (Fig. 5) nor fit the Langmuir equation, Table 2. Not surprisingly as some of the data points involved in the sorption isotherm, at such low pH values, represent a maximum chromium sorption (Fig. 1) showing high chromium uptake with very low equilibrium chromium concentration causing deviation from the L-type isotherm shape and the Langmuir equation. Langmuir parameters are shown in Table 2 together with related correlation values, [R.sup.2]. At low pH values, [R.sup.2] values are very low and a comparison of monolayer sorption capacity, q, cannot be established.

[FIGURE 5 OMITTED]

Kinetics of Cr(VI) removal and chromium sorption

As shown in the equilibrium studies, maximum sorption occurs at initial pH 2.8 for initial Cr(VI) concentration of 100 mg/l. Initial pH 2.8, 2.0 and 4.0, that represent the pH at, before and after maximum sorption for Cr(VI) solution (100 mg/l), respectively, were chosen for investigating the kinetics of Cr(VI) removal and total chromium sorption.

Cr(VI) removal at initial pH 2.0-4.0 is relatively slow reaching approximate equilibrium in 80-140 hours, Fig. 6. At initial pH 2.0 (Fig. 6A), Cr(VI) was found to decrease faster than total chromium with the progress of time. The difference between total chromium and Cr(VI) is apparently unadsorbed Cr(III) which was produced as a result of Cr(VI) reduction. By approaching equilibrium, the concentration of Cr(VI) further decreases and, by equilibrium, Cr(III) concentration increases to be the most abundant chromium species in solution, clearly identified by its greenish color.

At initial pH 2.8, the difference in concentration between Cr(VI) and total chromium decreased with time and, by approaching equilibrium, both concentrations overlapped, (Fig. 6B). Cr(III) produced slowly decreases, with the progress of time, almost to zero, as equilibrium approaches. At initial pH 4.00, a slight difference between Cr(VI) and total chromium developed in the first few hours, however with the progress of time, both concentrations overlapped showing almost no Cr(III) in solution (Fig. 6C). A drop in solution pH was observed in the early few hours of the kinetic experiment followed by a rise with the progress of time (Fig. 7). This could be related to the sorbent acidity resulted from the preparation conditions of the carbonaceous sorbent, which produced functional groups such as -COOH on the sorbent surface [20, 31].

[FIGURE 6 OMITTED]

pH rises significantly at initial pH 2.8 (Fig. 7) allowing the sorption of most Cr(III) showing higher chromium sorption (Fig. 8). However, the rise in pH was small at initial pH 2.0 showing less chromium sorption as protons compete with Cr(III) ions for binding sites (Fig. 8). The extent of Cr(VI) reduction decreased at initial pH 4.0 due to protons insufficiency leading to a decrease in chromium sorption. The presence of Cr(III) in solution in the very early stages of sorption at initial pH 4.0 could be related to the pH drop in solution, after mixing the sorbent, as a result of sorbent acidity. In addition, produced Cr(III) at such initial pH are most probably sorbed fast on the sorbent surface before being released in solution.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Cr(VI) removal data were found to fit well pseudo first order model (Eq. 1) at initial pH 2.0 with high correlation and deviates from fitting at initial pH 2.8 and 4.0, Fig. 9. By testing pseudo second order model (Eq. 2), the data did not fit at initial pH 2.0, but fit well at initial pH 2.8 and 4.0, Fig. 10. Since all the other experimental conditions are the same except the initial pH, the change in the order of Cr(VI) removal process is clearly related to the initial proton concentration. Considering the carbon concentration during the kinetic experiment as constant, being a solid material, the order of Cr(VI) removal process depends on [H.sup.+] and Cr(VI) concentrations. Ratio of [[H.sup.+]]/[Cr(VI)] at initial pH 2.0 is 5.2 showing that [[H.sup.+]] is high enough that can be considered as constant during the kinetic experiment. Thus, Cr(VI) removal process depends on Cr(VI) concentration and thus follows pseudo first order model (Eq. 1). However, at initial pH 2.8 and 4.0, the [[H.sup.+]]/[Cr(VI)] ratios are 0.82 and 0.052, respectively, showing that Cr(VI) removal is affected by the change in both Cr(VI) and [H.sup.+] concentrations with time. Thus, the process follows pseudo second order model (Eq. 2). In a previous study, the kinetics of Cr(VI) removal on modified flax shive [20] followed pseudo first order model (Eq. 1) at initial pH 1.5 and pseudo second order model (Eq. 2) at initial pH 4.5 [20].

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Chromium sorption data did not fit well pseudo first order kinetic model (Eq. 3) as presented in Fig. 11 and but fit well pseudo second order model (Eq. 4) at different initial pH values with higher [R.sup.2] values (Fig. 12). Obviously, sorption of chromium complies very well with pseudo-second order kinetic reaction which agreed with chemisorption as the rate-limiting mechanism through sharing or exchange of electrons between sorbent and sorbate [26,36]. In literature, Cr(VI) sorption followed pseudo first order model [11,31,37,38] while, other studies found that the process followed pseudo second order model [9,14,18,39,40]. Such variation in the sorption process order is probably related to the variation in the experimental conditions, sorbent types and the difference in their reactivity towards Cr(VI).

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Base neutralization

The sorbent, after the contact with 1000 mg/l of Cr(VI) at initial pH 1.8 until the equilibrium was reached, was separated and washed with 0.1 M [H.sub.2]S[O.sub.4], to strip metal ions from the sorbent surface. The sorbent samples were then washed by deionized water to become acid-free, dried at 105 [degrees]C and then used for base neutralization studies.

The differences of surface functionalities before and after the reaction with Cr(VI) were determined by Boehm titrations [28]. The three bases used in titration are considered as approximate probes for acidic functionalities: NaHC[O.sub.3] (carboxylic), Na2C[O.sub.3] (carboxylic and lactonic), NaOH (carboxylic, lactonic and phenolic). An increase in the concentration of carboxylic, lactonic and phenolic groups on the sorbent surface was found after the reaction with acidified Cr(VI), as a result of the oxidation processes occurring on the sorbent surface, Table 3. Possible surface oxidation processes are shown in equations 8-10.

~C-H + Cr (oxidized) + [H.sup.+] = ~C-OH + Cr (reduced) + [H.sub.2]O (8)

~C-H / ~C-OH + Cr (oxidized) + [H.sup.+] = ~C=O + Cr (reduced) + [H.sub.2]O (9)

~C-H / ~C-OH + Cr (oxidized) + [H.sup.+] = ~COOH + Cr (reduced) + [H.sub.2]O (10)

Conclusion

The carbonaceous sorbent prepared from palm leaflets, agricultural waste, via sulfuric acid treatment, possesses reduction properties for Cr(VI). Sorption and reduction of Cr(VI) depends on initial pH and Cr(VI) concentration. Sorption maxima appear as a result of Cr(VI) reduction with a significant rise in the solution pH that allows most of produced Cr(III) to be sorbed. Cr(VI) removal data, at initial pH 2.0, fit well pseudo first order model while at pH 2.8 and 4.0, the data fit well the pseudo second order model instead. The initial proton concentration between initial pH 2.0 and 2.8 seems responsible for the process order change. Sorption of total chromium follows pseudo second order process in the pH range (1.8-7) showing that the rate of the process depends on sorbent and sorbate concentrations. The prepared carbon from date palm leaflets with sulphuric acid treatment seems to be an efficient sorbent and reducing agent for Cr(VI) at lower pH values.

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* E.I. El-Shafey and H. Al-Lawati

Department of Chemistry, College of Science, Sultan Qaboos University, P.O. Box 36-Alkhodh 123, Muscat, Oman.

* Corresponding Author E-mail: dr_el_shafey2004@yahoo.co.uk
Table 1: Initial and equilibrium pH values at sorption
maxima using different initial Cr(VI) concentrations.

Initial Cr(VI) Initial pH Equilibrium pH Cr sorbed
 concentration (mg/l) (mg/g)

100 2.82 4.73 46.6
200 2.60 4.86 90.4
300 2.40 4.75 122.5
400 2.20 4.63 154.6
500 2.04 4.57 183.3

Table 2: Langmuir parameters for the chromium sorption at different
initial pH values in the concentration range of 100-500 mg/l.

pH q (mg/g) b(l/mg) [R.sup.2]

1.8 88.49558 0.007864 0.3242
2.0 108.6957 0.016853 0.3869
2.2 81.30081 0.244533 0.3834
2.4 161.2903 0.050243 0.5878
2.6 153.8462 0.067079 0.9947
2.8 147.0588 0.083847 0.9867
3.0 129.8701 0.041243 0.9764
4.0 128.2051 0.018723 0.9829
5.0 121.9512 0.013183 0.9631
7.0 121.9512 0.005795 0.9411

Table 3: Surface acidic functionality of the sorbent after the
reaction with acidified Cr(VI).

The dry sorbent Surface acidic functionalities based on weight
 (meq/g)

 Carboxyl (a) Lactone (a) Phenol (a)

Before reaction 1.6 1.32 1.08
After reaction 2.4 1.4 1.3

(a) NaHC[O.sub.3] (carboxyl), Na2C[O.sub.3] (carboxyl and lactone),
NaOH (carboxyl, lactone and phenolic)
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Author:Shafey, E.I. El-; Lawati, H. Al-
Publication:International Journal of Applied Environmental Sciences
Date:Nov 1, 2010
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